This invention relates to transmission distortion elimination devices for removing such distortion of a transmission path as ghost multiplexed with a television signal.
In receiving a television signal broadcasted by a television station, signal distortion of the transmission path caused by reflected waves which are generated by such obstacles as tall buildings and mountains and which are added to the direct wave is called "ghost", and it is a major cause of deteriorated picture quality in ground television broadcasting systems.
As an example of ghost elimination devices intended to remove the ghost, there is known a device which comprises a filter section which generates a ghost component, opposite in polarity to the ghost component added to the signal component, with a transversal filter and adds it to the original signal, and a controller which controls the tap coefficients of the transversal filter of the filter section, as described in the article "Ghost Canceller with GCR signal" of the Television Technology Report, Vol. 13, No. 32, pp. 31-36, published in June 1989.
An example of the method of controlling such tap coefficients by the controller is, as discussed in the above publication, an iterative correction method, in which a reference signal (GCR signal) for transmission distortion detection, such as for ghost, is introduced from the signal produced at the output of the filter section, the difference from a reference waveform which represents the characteristics of an ideal transmission path is evaluated thereby to detect the distortion, and the tap coefficients of the transversal filter are adjusted and corrected iteratively so that the difference becomes minimum.
The iterative correction method has typical algorithms known as MSE (Mean Square Error) method and ZF (Zero Forcing) method, and their tap coefficient correction is based on the following expression. EQU {Ki}.sub.t ={Ki}.sub.t-1 -a.multidot.{Ei}.sub.t ( 1)
where {Ei} is the value of the i-th tap correction: ##EQU1## where e.sub.i is the difference from the reference waveform.
{Ki} is the i-th tap coefficient, t is the number of iteration of correction and a is a constant (0&lt;a&lt;1).
According to the foregoing prior art, the tap coefficient is corrected iteratively in proportion to the amount of distortion (distortion value), and in consequence the tap coefficients which minimize the distortion are set on the transversal filter, whereby the television signal which is rid of such transmission distortion as ghost can be obtained at the output of the filter section. However, in the tap coefficient setting operation which takes place iteratively, an increased feedback value (constant a) at tap coefficient correction incurs the instability of control and susceptibility to such disturbing factors as impulsive noise, although it speeds up the correction, or a decreased feedback value results in an increased number of iteration and thus an extended time for the corrective operation.